Fiber-reinforced composites and method for the production thereof

ABSTRACT

A thread or other element for bonding fibers in a fibrous reinforcement for use in a fiber composite is such that it is at least degraded in the course of the production of said fiber composite.

The present application is a divisional of U.S. application Ser. No.10/654,929, filed Sep. 5, 2003, which in turn claims benefit of acontinuation of U.S. application Ser. No. 09/585,284, filed Jun. 2,2000, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a fibrous reinforcement for use in fibercomposites wherein crossover points of fibers are bonded together usingthreads or other elements. The invention relates also to a process forproducing fiber composites comprising a matrix material and a fibrousreinforcement embedded in the matrix material, wherein the fibers in thefibrous reinforcement are bonded together using threads or otherelements.

Fiber composites are materials comprising a matrix material, forexample, a polymer matrix as a continuous phase, and embeddedreinforcing fibers as a discontinuous phase. The reinforcing fibersbound into the matrix material improve the properties of the matrixmaterial, i.e., of the plastic. More particularly, the strength andstiffness of the material can be advantageously modified.

The fibrous reinforcement used is typically in the form of a sheetlikearrangement of fibers. Sheetlike arrangements of fibers are also knownas textile sheet materials or fabrics and can be subdivided into threegroups:

-   a) webs;-   b) noninterlooping systems (wovens, scrims, braids);-   c) interlooping systems (loop-formed knits, loop-drawn knits).

Depending on the method used to construct or produce these sheetmaterials, the fixation of the actual reinforcing fibers takes differentforms. Owing to their structure, for example, loop-drawn knits,loop-formed knits and wovens normally require no additional fixation.Webs, in contrast, may be needled or fixed (bonded) by means of abinder. Nonwoven scrims, which are formed by superposition ofdifferently oriented fiber or filament systems, require fixation of thecrossover points of individual fibers to be handleable.

After impregnation and curing or solidification of the matrix material,the “thread” (which, as used herein, can include other fixationelements) is still present in fiber form in the composite as anadditional phase and accordingly possesses an interface with the matrixmaterial.

It has been determined that the thread may be an imperfection and apossible initiation point for damage, owing to the different physicalproperties of the thread material, compared with the matrix material andcompared with the actual reinforcing material, and owing to possiblyinadequate attachment of the thread to the matrix material. If, as aresult of the thread having an interfacial area, the surface of thestitching thread becomes detached from the matrix material, it is likelythat, similarly to other pores, dynamic stress on the fiber compositewill lead to areas of damage emanating from the thread and possibly tocomplete failure of the fiber assembly.

Furthermore, matrix materials where the infiltration or curing requiresthat they be exposed to an elevated temperature above room temperatureor above the storage temperature of the fibrous reinforcement areproblematical in that, beyond a certain temperature, the shrinkagetendencies of the thread will be responsible for distortions ordistensions being introduced into the actual reinforcing fibers as afunction of the mechanical and/or thermal processing history of thethread. This risk of the fibrous reinforcement being distorted is alwayspresent when the thread undergoes shrinkage prior to the actual gellingor solidifying of the matrix material.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide animproved fibrous reinforcement for use in producing fiber-reinforcedcomposites.

Another object of the invention is to provide improved fiber-reinforcedcomposites.

It is also and object of the invention to provide a process or methodfor producing the improved fiber-reinforced composites.

In accomplishing the foregoing objects, there has been providedaccording to one aspect of the present invention a fiber-reinforcedcomposite material, comprising a solid matrix material having dispersedtherein a fibrous reinforcement, wherein the fibrous reinforcementcomprises a plurality of fibers that were at one time bonded together atspaced points by at least one bonding element, wherein the bondingelement is at least substantially degraded in the course of making thecomposite material, at the latest upon the solidification of the matrixmaterial. Preferably, degradation has occurred to the extent that thereis substantially no distortions or distensions introduced into thecomposite material in the course of the manufacturing process, and mostpreferably to the extent that there is substantially no interfacial areabetween the bonding element and matrix material in the fiber-reinforcedcomposite material.

In accordance with another aspect of the invention, there has beenprovided a fibrous reinforcement for use in forming a fiber-reinforcedcomposite material having the fibrous reinforcement encapsulated withina solidified matrix material, wherein the fibrous reinforcementcomprises a plurality of fibers that are bonded together at spacedcrossover points by at least one bonding element that is at leastsubstantially degraded in the course of making the composite material,at the latest upon the solidification of the matrix material.

In accordance with yet another aspect of the invention, there has beenprovided a process for producing a fiber-reinforced composite materialhaving the fibrous reinforcement encapsulated within a solidified matrixmaterial, comprising:

-   (a) placing into a mold cavity a fibrous reinforcement comprising a    plurality of fibers that are bonded together at spaced points by at    least one bonding element;-   (b) injecting into the mold cavity the matrix material or a    precursor to the matrix material, in flowable form, to surround the    fibrous reinforcement with the matrix material; and-   (c) solidifying the matrix material to form the composite material,    -   further comprising, after step (a), at least substantially        degrading the bonding element in the course of making the        composite material, at the latest upon the solidification of the        matrix material.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description of preferred embodimentsthat follows.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the invention, threads or other bonding elements are atleast substantially degraded in the course of the production of thefiber-reinforced composite. One possibility is to decompose the threadsafter the fibrous reinforcement has been incorporated into the matrixmaterial. The only purpose of the threads, therefore, is to render thefibrous reinforcement conveniently handleable prior to encapsulation inthe matrix material.

The degrading or decomposing of the threads may be accomplished by theappropriate choice of the thread material, so that the threads dissolvein the matrix material. For each matrix material, it is thus necessaryto select a suitable thread material which, in addition, has sufficientmechanical strength to initially fix or hold together the reinforcingfibers of the fibrous reinforcement.

It is also conceivable to select a thread material such that the threadsmelt at a temperature which is above the room or storage temperature ofthe fibrous reinforcement and which can be as high as the temperaturefor manufacturing the fiber composite. The melting point of the threadscan be, for example, equal to the curing temperature of the matrixmaterial or equal to the temperature for impregnating the fibercomposite.

Thread materials which melt at the impregnation temperature areadvantageously copolymers, especially copolymers based on polyamide orpolyester.

In a corresponding process for producing fiber-reinforced composites,the threads degrade when the fibrous reinforcement is bound orincorporated into the matrix material.

As a result of the thread dissolving in the matrix material, the threadis not present in the fiber composite as a separate phase. It canaccordingly not act as an imperfection and as a possible initiationpoint for damage, which is the case with conventional threads thatpossess an interface with the matrix material. Furthermore, thedissolving of the thread avoids distortions or distensions of thefibrous reinforcement of the kind which can arise in the case ofconventional threads as a result of the shrinkage tendencies of thethreads at elevated manufacturing temperatures of the fiber composite.This benefit is achieved already if the thread is at least degraded,i.e., not completely decomposed.

Furthermore, appropriate compatibility of the stitching thread materialwith the matrix material makes it possible to improve the materialcharacteristics of the fiber composite under impact stress.

The invention will now be more particularly described with reference tospecific aspects and examples.

To improve handling, fibrous reinforcement is bonded together atcrossover points of individual fibers using bonding elements, preferablyin the form of threads. This makes it possible to fix the fibrousreinforcement in certain shapes and to drape the fibrous reinforcementin corresponding fashion. In addition, such a thread-fixed nonwovenscrim of fiber mats is easier to transport as a result of the crossoverpoints being fixed or bonded together. Thus, various types of bondingelements can be used, including, for example, adhesive spots, staples,docking holes, and the like.

To produce fiber composites, preferably a thread-fixed fibrousreinforcement is introduced into a matrix material. The matrix materialcan be, for example, a thermosetting binder such as PF(phenol-formaldehyde), MF (melamine-formaldehyde), UF(urea-formaldehyde), UP (polyester) and EP (expoxide) resins. It is alsopossible to use thermoplastic binders, such as PA (polyamide), PC(polycarbonate), POM (polyoxymethylene), PET (polyethyleneterephthalate), PBT (polybutylene terephthalate), PP (polypropylene) andABS (acrylonitile-butadiene-styrene copolymer) resins. Embedding thefibrous reinforcement in these polymer materials improves especially themechanical and thermal properties of the base material, for example, thetensile and breaking strength, the modulus of elasticity, the heatresistance and the dimensional stability. The improvement in theseproperties which is attainable in any particular case depends on theadhesion between the matrix material and the fibrous reinforcement,i.e., on the processes at the boundary layer. It is therefore proposedthat the threads which are necessary for fixing the fibrousreinforcement are at least degraded and are preferably decomposed in thefiber composite and thus are no longer able to form a boundary layer.

One way for the threads to be decomposed is for them to dissolve in thematrix material. It is also advantageous to use combinations of threadmaterials with matrix materials in which the thread materials melt atthe curing temperature of the matrix material. It is furtheradvantageous to use combinations of thread materials and matrixmaterials where the thread material melts at the temperature forimpregnating the fiber composite.

In one preferred embodiment of the invention, fiber-reinforcedcomposites are produced utilizing a multi-axial fabric (non-scrimmedfabric) made of carbon fibers having a basis weight of about 1000 g/m².The layers of fiber forming the fabric are stitched using thin threads(about 70 dtex). The thin threads are made of copolyester K-110 materialavailable from EMS-Chemie of CH-7013 Donat/Ems. The melting temperatureof the thread material is about 120° C. Upon incorporation into a highermelting matrix polymer, the copolyester threads are melted and areintimately combined with the matrix material and are thereforeessentially dissolved.

In accordance with another preferred embodiment of the invention, asimilar fabric was employed as the reinforcing material, except in thiscase the threads used to stitch the fabric were made of polyvinylalcohol (PVAL), which is water soluble and is washed out of the fabriclayer by a washing step after it is inserted into the mold but beforethe matrix material is injected.

Although the invention has been described and illustrated with referenceto preferred embodiments thereof, it will be appreciated by thoseskilled in the art that the invention can be practiced in the form ofother embodiments. It is intended that all obvious equivalents to thedisclosed embodiments will be covered by the appended claims.

The entire disclosure of German Patent Application No. 199 25 588.1-43,filed Jun. 4, 1999, is hereby incorporated by reference into the presentapplication.

1. A fiber-reinforced composite material, comprising a solid matrixmaterial having dispersed therein a fibrous reinforcement, wherein thefibrous reinforcement comprises a plurality of fibers that were at onetime bonded together at spaced points by at least one bonding element,wherein the bonding element is at least substantially degraded in thecourse of making the composite material, at the latest upon thesolidification of the matrix material.
 2. A fiber-reinforced compositematerial according to claim 1, wherein the bonding element has been atleast partially dissolved in the matrix material.
 3. A fiber-reinforcedcomposite material according to claim 1, wherein the bonding element hasbeen at least partially melted at the curing temperature of said matrixmaterial.
 4. A fiber-reinforced composite material according to claim 1,wherein the bonding element has been at least partially melted at atemperature at which the fibrous reinforcement has been impregnated withthe matrix material in liquid form.
 5. A fiber-reinforced compositematerial according to claim 1, wherein the bonding element is liquidsoluble and has been dissolved from the fibrous reinforcement in a stepprior to impregnating the fibrous reinforcement with the matrixmaterial.
 6. A fiber-reinforced composite material according to claim 5,wherein the liquid comprises an aqueous liquid.
 7. A fiber-reinforcedcomposite material according to claim 1, wherein the bonding elementcomprises a thread.
 8. A fiber-reinforced composite material accordingto claim 1, wherein the bonding element is completely structurallydestroyed in the final composite material.
 9. A fiber-reinforcedcomposite material according to claim 1, wherein the bonding elementcomprises a copolymer.
 10. A fiber-reinforced composite materialaccording to claim 9, wherein the copolymer comprises a polyamide or apolyester.
 11. A fiber-reinforced composite material according to claim1, wherein the matrix material comprises a polymer.
 12. Afiber-reinforced composite material according to claim 11, wherein thematrix polymer is selected from the group consisting of PF(phenol-formaldehyde), MF (melamine-formaldehyde), UF(urea-formaldehyde), UP (polyester), EP (expoxide), PA (polyamide), PC(polycarbonate), POM (polyoxymethylene), PET (polyethyleneterephthalate), PBT (polybutylene terephthalate), PP (polypropylene) andABS (acrylonitile-butadiene-styrene copolymer) resins.
 13. A fibrousreinforcement for use in forming a fiber-reinforced composite materialhaving the fibrous reinforcement encapsulated within a solidified matrixmaterial, wherein the fibrous reinforcement comprises a plurality offibers that are bonded together at spaced crossover points by at leastone bonding element that is at least substantially degraded in thecourse of making the composite material, at the latest upon thesolidification of the matrix material.
 14. A fibrous reinforcementaccording to claim 13, wherein the matrix material comprises a polymericmaterial and the bonding element comprises a polymeric material that hasa melting temperature lower than either the melting temperature of thematrix material or a curing temperature for solidifying the matrixmaterial.
 15. A fibrous reinforcement according to claim 13, wherein thebonding element comprises a polymeric material that is water soluble.16. A fiber-reinforced composite material according to claim 1, whereinthe composite material is made by a process comprising: (a) placing intoa mold cavity a fibrous reinforcement comprising a plurality of fibersthat are bonded together at spaced points by at least one bondingelement; (b) injecting into the mold cavity the matrix material or aprecursor to the matrix material, in flowable form, to surround thefibrous reinforcement with the matrix material; and (c) solidifying thematrix material to form the composite material, wherein, after (a), atleast substantially degrading the bonding element in the course ofmaking the composite material, at the latest upon the solidification ofthe matrix material.
 17. A fiber-reinforced composite material having afibrous reinforcement encapsulated within a solidified matrix materialproduced by a method, comprising: (a) placing into a mold cavity afibrous reinforcement comprising a plurality of fibers that are bondedtogether at spaced points by at least one bonding element; (b) injectinginto the mold cavity the matrix material or a precursor to the matrixmaterial, in flowable form, to surround the fibrous reinforcement withthe matrix material; and (c) solidifying the matrix material to form thecomposite material, wherein, after step (a), at least substantially butonly partially degrading the bonding element in the course of making thecomposite material, at the latest upon the solidification of the matrixmaterial, the degrading being carried out to at least an extent thatthere is substantially no interfacial area between the bonding elementand matrix material in the fiber-reinforced composite material.